1. Field of the Invention
The present invention relates to laminated glazings and, more particularly, to laminated safety glass having an interlayer including a plasticized polyvinyl chloride resin. The laminated safety glass of the invention is particularly useful for producing automotive and airplane windshields, and architectural glazings.
2. Summary of Related Art
Safety glass is a well known term for a glass sandwich composed of an interlayer bonding together two glass plates or sheets so that breaking the glass results in minimum dispersion of broken glass fragments. The interlayer must possess a number of properties, including the following: high impact energy adsorption to minimize concussive injury; shear and tear strength sufficient to prevent rupture of the interlayer by the broken glass; sufficient adhesion to the glass to inhibit laceration on contact with, and prevent dispersion of, broken glass; acceptable thermal and light stability; acceptable weatherability; and good optical quality. The interlayer must possess these properties over the wide range of temperatures in which these laminated glazings are used for relatively long periods of time.
It has been widely known to use a film of plasticized polyvinyl butyral as the interlayer material in safety glass for automobiles, air planes and building materials because of its high adhesiveness and superior light stability, transparency and low-temperature flexibility. However, the use of plasticized polyvinyl butyral films makes the production of laminated safety glass relatively expensive.
The surface of plasticized polyvinyl butyral film is very tacky, and presents a problem of blocking at the time of windup after film formation. Thus, plasticized polyvinyl butyral film must be provided with some parting means if it is to be stored or transported in the form of stacks of die-cut blanks or in the form of rolls. Furthermore, the production of plasticized polyvinyl butyral films requires specialized equipment and, due to their sensitivity to moisture, plasticized polyvinyl butyral films must generally be handled under controlled atmosphere conditions during manufacture, storage and immediately prior to their incorporation into the laminated safety glass. This all adds to the expense of utilizing plasticized polyvinyl butyral films in laminated safety glass.
Alternative interlayer materials have been proposed. For instance, U.S. Pat. No. 4,277,538 to Beckmann et al discloses a laminated safety glass employing a sheet of plasticized polyvinyl chloride (PVC) as the interlayer. The use of PVC would be advantageous in that it may be produced on conventional equipment and would be much less expensive to manufacture and process into a laminated safety glass when compared with polyvinyl butyral. However, by itself, a PVC film will not adhere to glass. To increase the adhesion of the PVC to glass, Beckmann et al suggest the use of an organofunctional silane, either as a primer or uniformly dispersed within the PVC film.
At least certain of the silicon-organofunctional silanes proposed by Beckmann et al are effective to increase the adhesion of a PVC interlayer to the glass sheets of a laminated safety glass. However, the laminated safety glass employing a PVC interlayer as taught by Beckmann et al does not possess all of the properties required of an interlayer for laminated safety glass, and therefore has not been placed into commercial use. For instance, the Beckmann et al interlayer has poor long term heat and light stability at service use temperatures. With only a few years of exposure to normal use levels of heat and light, the PVC interlayer tends to degrade to the point where it has visibly yellowed and is aesthetically unacceptable, and the haze has increased to the point that visibility through the glazing is significantly impaired. It would therefore be advantageous to develop a laminated safety glass with a PVC interlayer wherein the interlayer exhibits a lower level of degradation over its useful life.